In manufacturing integrated circuits, various thin films are deposited and patterned on a semiconductor substrate. One well-known deposition process is chemical vapor deposition (CVD). In general, CVD is a process in which a substrate, e.g., a semiconductor workpiece, is heated and coated with layers of volatile chemical compounds. During CVD, a precursor compound is reduced or dissociated in a chemical reaction on or adjacent to the substrate surface. The precursor compound is delivered to the reaction chamber in a vapor or gaseous state, often in a reaction gas which also includes a suitable carrier gas. In reactive CVD processes, the reaction gas may also include other compounds, e.g., other precursors, which react with one another. These CVD processes yield an adherent coating on a surface of the substrate.
CVD processes are used at a number of stages in semiconductor manufacture. For example, CVD can be used to produce epitaxially grown single crystal silicon by the reduction of a silicon precursor, such as silicon tetrachloride, by a reactant gas such as hydrogen. This process is used to make other epitaxial layers such as polysilicon, silicon nitride, silicon dioxide, and both doped polysilicon and silicon dioxide. CVD is also used in semiconductor manufacture to depositing various conductive films, such as aluminum and copper.
One advantage of CVD processes is that the deposited film is highly conformal, i.e., it yields a uniform film even on more complex surfaces with high aspect ratio features. As these features become smaller and ever more densely packed together, though, it is difficult to achieve uniform step coverage even with CVD. Atomic layer deposition (ALD, also referred to as atomic layer epitaxy) is an improvement of conventional CVD processes. Rather than continuously depositing material to build up a coating, ALD rapidly deposits a series of monatomic layers atop one another. Though ALD is materially slower than CVD, it improves conformality of the film.
One limitation on the variety of films which can be deposited via CVD and ALD is that the reaction precursor must be delivered in a gaseous state. Many potentially useful reaction precursors are solids or relatively low vapor pressure liquids. It can be difficult to volatilize such precursors at a rate fast enough for a commercially acceptable production throughput. It can also be difficult to maintain a consistent delivery rate of the precursor gas over the course of a deposition process, with the delivery rate often decreasing as the deposition process proceeds. If the precursor is a solid, it typically must be sublimated, which often makes it more difficult to produce the gaseous precursor at an acceptable, stable rate. This can sometimes be attributed to agglomeration from a particulate solid precursor, which reduces the effective surface area of the precursor over time, driving down the rate of volatilization.